Why some mammalian species choose to spend their lives with the same mates has long baffled scientists — and will probably continue to do so as two new massive studies present contradictory results.

One group of researchers says monogamy evolved in primates to counter the threat of males killing babies to boost their siring success. The other team concludes that mammals, including primates, become monogamous when females live far away from one another.

The differences in the studies have raised eyebrows. “They do seem to be saying the opposite thing,” says Anthony Di Fiore, an evolutionary anthropologist at the University of Texas at Austin. “It’s interesting because they use very, very similar methods,” Di Fiore says.

The two groups also disagree on whether the research has implications for why humans evolved fidelity to mates.

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